The invention relates to aqueous inkjet inks with oil-soluble dyes that are made more soluble in the aqueous vehicle by making a microemulsion in the ink of solvent moieties with both solubilizing groups and hydrophobic groups.
The use of inkjet printing systems has grown dramatically in recent years. This growth may be attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost. Today""s inkjet printers offer acceptable print quality for many commercial, business, and household applications at costs fully an order of magnitude lower than comparable products available just a few years ago. Notwithstanding their recent success, intensive research and development efforts continue toward improving inkjet print quality, while further lowering cost to the consumer.
An inkjet image is formed when a precise pattern of dots is ejected from a drop-generating device known as a xe2x80x9cprintheadxe2x80x9d onto a printing medium. The typical inkjet printhead has an array of precisely formed nozzles located on a nozzle plate and attached to an inkjet printhead substrate. The substrate incorporates an array of firing chambers that receive liquid ink (colorants dissolved or dispersed in a solvent) through fluid communication with one or more ink reservoirs. Each chamber has a thin-film resistor, known as a xe2x80x9cfiring resistor,xe2x80x9d located opposite the nozzle so ink can collect between the firing resistor and the nozzle. In particular, each resistor element, which is typically a pad of a resistive material, measures about 35 xcexcmxc3x9735 xcexcm. The printhead is held and protected by an outer packaging referred to as a print cartridge, i.e., inkjet pen.
Upon energizing of a particular resistor element, a droplet of ink is expelled through the nozzle toward the print medium, whether paper, transparent film or the like. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements, thereby forming alphanumeric and other characters on the print medium.
Water is the most attractive solvent for inkjet ink. Because of this, drop-on-demand inkjet inks are 50-90% water. Accordingly, most of the inkjet ink formulations are based on water-soluble colorants, that is, water-soluble dyes or water-dispersible pigments. This comes at the price that inkjet images are not stable to water, in particular, to the action of water combined with rubbing (so-called wet smudge).
On the other hand, there are many inks in the art that are stable to the action of water. These inks are based on organic solvents and use solvent-soluble dyes or solvent-dispersible pigments. These inks are widely used in such areas as continuous inkjet and gravure printing inks. The solvent systems used in these applications are methylethylketone, methylbutylketone, acetone, toluene, ethyl acetate, as well as solvent mixtures, such as 40% ethyl acetate, 40% ethyl alcohol, and 20% toluene. These solvents however are hard to use in drop-on-demand inkjet. The main reasons for this are: (1) The high activity of the solvents towards the printhead materials; and (2) objectionable smell and toxicity. In addition, for bubble-jet (but not for piezo inkjet), there are the additional problems in these solvent systems of (1) poor bubble nucleation and (2) inadequate temperature control, both of which lead to overheating the printheads. Moreover, when the solvent-based inks are not dried quickly by evaporation, they tend to penetrate deep into the paper, thereby deteriorating the image quality.
An approach to improve the attributes described above are various xe2x80x98emulsionxe2x80x99 techniques. Emulsions represent a plurality of droplets of one liquid in another. Emulsions can be of two different types, e.g., oil-in-water (O/W) or water-in-oil (W/O). In addition to this, emulsions can be microemulsions and macroemulsions. Microemulsions are thermodynamically equilibrium systems. Some microemulsions are also known as xe2x80x9cswollen micellesxe2x80x9d, which means that they represent surfactant micelles with the oil solubilized inside. Typical microemulsion particle size is 5-50 nm. Microemulsions are normally transparent or slightly bluish because of the very small particle size. On the other hand, macroemulsions are not in thermodynamic equilibrium and do not form spontaneously, that is, they require mechanical agitation for preparation. Furthermore, their particle size is larger, 50 nm and up. One of the common examples of macroemulsions is milk. Finally, xe2x80x98miniemulsionsxe2x80x99 are a special case of macroemulsions with smaller particle size, below 1 xcexcm. They are not thermodynamically stable and do not form spontaneously; however, they can be made kinetically stable, so that the particle size does not change with time.
The use of microemulsions in inkjet ink has been known in the art for several years. Thus, in the patent of Wickramanayake et al., (U.S. Pat. No. 5,226,957) entitled xe2x80x9cSolubilization of Water-Insoluble Dyes via Microemulsions for Bleedless, Non-Threading, High Print Quality Inks for Thermal Inkjet Printersxe2x80x9d, it is suggested to incorporate an oil-soluble dye into a microemulsion, which contained solvents, surfactants, co-surfactants, and water. In another patent of Wickramanayake (U.S. Pat. No. 5,565,022, xe2x80x9cFast-Drying, Bleed-Free Inkjet Ink Compositionsxe2x80x9d), it is suggested to incorporate a dye into a mixture of an organic solvent, surfactant, and water, so that the dye and water-insoluble solvent are solubilized by the surfactant. In U.S. Pat. No. 5,643,357 (Brenton et al., xe2x80x9cLiquid Crystalline Ink Compositionsxe2x80x9d), an ink composition is suggested, that contains a surfactant, and oil-or-alcohol-soluble dye, and water. The formulation undergoes a phase transition upon heating from a microemulsion phase to a lamellar phase, which helps to stratify the inks to the surface of the paper, when the printing substrate is heated. In the U.S. Pat. No. 5,492,559 (Oliver et al., xe2x80x9cLiquid Crystalline Microemulsion Ink Compositionsxe2x80x9d), an ink formulation is described containing an aqueous phase, oil phase, and surfactant, and an oil-soluble dye, such that the system undergoes a microemulsion-liquid crystalline state transition with decreasing temperature. The idea is (with some variations) to solubilize water insoluble xe2x80x9csolventxe2x80x9d dyes in micelles, and thereby, produce water and smudgefast images.
All the known microemulsion formulations fall short of attaining a good print quality because of the excessive ink penetration into the paper and wicking along the fibers. Also, the need remains in solvents that have less smell, lower toxicity, and higher solubilizing capacity towards the solvent dyes. Finally, the need remains for a solvent system that combines all these attributes together, which is not a trivial task.
A major formulation problem involved in this approach is the limited solubility of the solvent dyes in the microemulsion formulations. Indeed, in order to get the full benefits of water-based inks, that is, the good bubble nucleation and temperature control, low toxicity and smell, and moderate viscosity, the weight percentage of the solvent in the ink should not exceed 50%; more preferably, it should not exceed 20%. If one further assumes that the solvent dye is completely water-insoluble and dissolves only in the solvent domains of the microemulsion, then, in order to get the dye load of 3%, typical for drop-on-demand inkjet, one needs the solubility of the dye in the solvent to be at least 6%, preferably at least 15%. Even higher solubility is in fact desired to prevent the spontaneous crystallization of the dyes from the solution caused by temperature variations. 20-50% solubility of the dye is highly desirable. However, most solvent dyes cannot be dissolved to such a degree in apolar oils conventionally used in microemulsion formulations. The reason for this is the fact that most of the solvent dyes conventionally used in inkjet inks are not soluble in both water and hydrocarbons.
The present invention relates to an aqueous inkjet ink composition comprising:
(a) dye molecules;
(b) micelles comprising a solvent of a formula Rxe2x80x94S, S being a dye-solubilizing group and R being a hydrophobic group; and
(c) an aqueous vehicle,
wherein the micelles are surrounded by the aqueous vehicle, each micelle having a monolayer surface enclosing an inside area of the micelle, the hydrophobic groups (R) extending into the inside area of the micelle from the monolayer surface and the dye-solubilizing groups (S) being at the monolayer surface of the micelles, and the dye molecules associating with the dye-solubilizing groups (S) at the monolayer surface of the micelles, the micelles separating the dye particles from the aqueous vehicle.
The present invention further relates to a method of making an aqueous inkjet ink composition comprising the steps of:
a) forming micelles in an aqueous vehicle by adding chemical compounds to an aqueous vehicle, the chemical compounds comprising a solvent of a formula Rxe2x80x94S, R being a hydrophobic group and S being a dye-solubilizing group, each micelle having a monolayer surface enclosing an inside area of the micelle, the hydrophobic groups (R) extending into the inside area of the micelle and the dye-solubilizing groups (S) being at the monolayer surface of the micelle; and
b) solubilizing dye molecules in the aqueous vehicle by adding dye molecules to the aqueous vehicle of a), the dye molecules associating with the dye-solubilizers at the monolayer surface of the micelles, the micelles separating the dye particles from the aqueous vehicle.